Sperm motility inhibitor from human seminal plasma: presence of a precursor molecule in seminal vesicle fluid and its molecular processing after ejaculation

Human seminal plasma contains a protein factor that has the capacity to inhibit the movement of demembranated and intact spermatozoa. This factor ‘seminal plasma motility inhibitor’ (SPMI) has been shown to originate exclusively from the seminal vesicles. The present results demonstrate that the biological activity of SPMI in semen decreases rapidly from 1000 U/d, immediately after ejaculation, to 220 U/ml 2 h later. Immunoblots of seminal plasma proteins probed with an antibody against human SPMI, revealed the rapid processing of a predominant 52 kDa SPMI antigen, present in the seminal vesicle secretions. This precursor was degraded initially into intermediate molecular mass fragments of 25–40 kDa, and subsequently into smaller fragments of 17–21 kDa. When seminal vesicle fluid was mixed with prostatic secretions (3: 1 v/v), proteases present in prostatic secretions were shown to be responsible for processing of the SPMI precursor. Addition of protease inhibitors such as phenylmethylsulphonyl fluoride (PMSF, 5 mM), benzamidine (100 mM) or ethylenediaminetetraacetic acid (EDTA, 5 mM) to the mixture of seminal vesicle and prostate secretions partially prevented the loss of activity of SPMI by 54%, 27% and 9%, respectively. However, the simultaneous addition of PMSF and benzamidine conferred almost total stability to the SPMI precursor activity. These results demonstrate that SPMI exists as a predominant 52 kDa precursor form in the seminal vesicles and is processed rapidly after ejaculation into less active, lower molecular mass forms by one or more serine proteases and/or metallo‐proteases of prostatic origin which are present in liquefied semen.